1. Field of the Invention
The present invention relates to a system for repairing a hole of significant size in a generally planar sheet member, particularly in an instance in which the sheet member may be comprised of wood, metal, ceramic, or of a composite material.
2. Description of the Prior Art
It is known to fabricate structural panels from a composite material comprising layers of reinforcing fibers (e.g. graphite fibers) within a resin matrix. The use of such composite materials for making structural and nonstructural components is well-known and widely recognized for its benefit of providing high strength with low weight since a construction is applicable to vehicles of all varieties, such as ground and water vehicles, aircraft and even space vehicles, and also include fixed structures such as buildings and storage tanks. When damage occurs to a structural panel of this material, the damage must be repaired to restore the structural strength of the panel, restore its smoothness and appearance, and in the case of aerodynamic surfaces, reconfigure the surface to its original curvature or configuration, as close as possible to the original mold line.
The problem is particularly troublesome with modern composite materials because of the higher load requirements for these materials and because of the necessity for producing flush repairs without the use of fasteners. Fasteners had previously been permitted for applying patches but undesirable for repairing modern composite structures. To accommodate the fasteners, additional holes must be drilled in the material, which creates additional weakness in the panel. Moreover, it is difficult to produce a patch that is flush with the original mold line of the panel and conforms exactly with its curvature when using a patch secured with fasteners. Finally, such fasteners exert a constant compression force on the repaired composite panel or honeycomb panel to hold the patch in place which could distort the cross section of the panel in the patch location and thereby reduce its load carrying capacity.
The installation of prior art patches, in the vicinity of underlying spars and ribs within the member, sometimes on the blind-side of the panel, required specially designed patches or connecting structure to the struts or ribs which could interfere with underlying mechanism. On panels which form an aerodynamic surface, especially on high performance aircraft, the non-conformance to the original contour and loss of smoothness could produce a flutter of the control surface which could adversely affect aircraft performance and cause premature fatigue delaminating of the panel structure.
Sometimes, prior art patches for composite materials required a series of steps, each requiring a separate cure time for the bonding agent in each of the steps. This prolonged the repair process and increased the number of steps involved in making the repair, thereby increasing the cost and increasing the down time of the equipment being repaired. In addition, it is an exacting process requiring special skills and care to produce an acceptable patch. Moreover, the prior art patches have required complicated, expensive, and cumbersome equipment requiring special training of operators and subject to failure in operation which would often interrupt the capability of the facility to make those repairs. Some other unsolved problems with prior art patches have included the inability to effect a permanent hermetic seal against leakage of pressurized fuel, air, water and other fluids and gases through the patch. Also, prior art patches were often thermally or chemically incompatible with the parent structure so that differential rates of thermal expansion caused loss of hermetic integrity of the patch or cause the panel to become distorted. Chemical incompatibility could cause loss of chemical resistance to certain chemicals such as hydraulic fluid which could cause the patch to swell and weaken. In addition, dissimilar metals and carbon/metal combinations can cause a harmful galvanic reaction that would weaken the patch.